In spite of its abundant supply in nature, chitin has not been utilized to the fullest extent and its effective utilization is strongly desired. Many proposals have been made for possible applications of chitin and chitosan; see, for example, Kagaku to Seibutsu (Chemistry and Biology), Vol. 21, p. 635 (1983). However, very few cases of successful commercialization of chitin and chitosan have been reported, primarily because of the non-existence of established technology for shaping these polysaccharides. For example, in order to exploit the metal adsorbing and ion exchange capabilities of chitosan on an industrial scale, efficient techniques for shaping and processing chitosan are necessary, but no such techniques have yet been established in the art.
The following comparisons can be made between shaped bodies of chitin and chitosan. First, chitin is stronger than chitosan, whereas the latter has more versatile capabalities, for example, ion exchange, metal adsorption, and antifungal properties. The solubility of chitosan in aqueous acid solutions proves advantageous for the purpose of shaping and processing chitosan, but has turned out to be a serious problem hindering commercial application of shaped chitosan bodies. It has therefore been strongly desired to develop a practical shaped body that has capabilities comparable to those of chitosan and yet does not dissolve in acid.
One of the greatest concerns in the fields of plastic surgery and dermatology is the development of effective methods for healing skin defect wounds such as burns and lesions left after skin excision. Wound dressings are indispensable for this purpose and a lot of materials have been investigated. Synthetic wound dressings so far proposed include specially woven fabrics of nylon, polyester, polypropylene or rayon (wherein monofilaments with a diameter of several hundred microns form surface projections in the form of loop piles or nappings), laminations of these fabrics and a silicone sheet, sponges of formylated polyvinyl alcohol, gels of a block copolymer of ethylene oxide and polyoxypropylene glycol, and "Sofratulle" (a gauze available from Nippon Roussel, impregnated with an antibiotic containing ointment). Wound dressings made of biological materials include nonwoven fabrics made of fibrin membrane, plasma membrane, and even collagen fibers (such nonwoven fabrics are sold, e.g., by Meiji Seika, Ltd. under the trademark "MEIPAC"), as well as porcine skins prepared by sterile freeze-drying of the dermis of the back of pigs. Several of the wound dressings mentioned above are described in Geka-Shinrvoshi (Journal of Surgical Diaqnosis), February 1975, p. 121.
However, none of the wound dressings proposed so far are completely satisfactory as protective materials for skin defect wounds. The general functions of wound dressings are the prevention of any loss of body tissues from the wound surface, prevention of infection, and assisting in the formation of granulation and epidermis. In order to retain these functions until a regenerating skin forms, ideal wound dressings must satisfy various requirements such as high strength, good adhesion to the lesion, high water abscrption, high adsorbability of exudates, high moisture permeability, histocompatibility, the absence of antigenicity, and the ability to promote skin regeneration. Nevertheless, the existing wound dressings made of synthetic materials are low in water absorption, adsorbability of exudates, and moisture permeability, whereas wound dressings made of naturally occurring substances are particularly low in strength and adsorbability of exudates, and are readily liquefied so as to become pulpy in only a few days after being applied to deep wounds. As a result, the conventional synthetic or natural wound dressings used to protect wounds have been unable to realize fully satisfactory formation of the granulation or epidermis. Burns are divided into the following four degrees in increasing order of severity: first degree, superficial second degree, deep second degree, and third degree burns. Existing wound dressings have been applied to the therapy of first degree burns and superficial second degree burns exclusively. In a similar manner, lesions that are left after excising split thickness grafts are the only wounds that can be treated at all by the conventional wound dressings.